1. Field of the Invention
The invention relates to cathode ray tube displays. More specifically, the invention describes a method and apparatus for generating a dynamic rotation field to precisely align raster scan lines displayed on a cathode ray tube. The invention is of significant interest in low and very low power sinusoidal, wide and very wide angle deflection systems, as well as DTV, HDTV and VHDTV, and other high and very high resolution video displays.
2. Definitions
To align is to cause a video image to be adjusted so that distortion characteristics are minimized and the video image that is displayed on the cathode ray tube forms an image that is pleasing to the eye.
A cathode ray tube refers to the tube structure, the phosphor screen, the neck of the tube, the deflection and control windings, including the yoke, rotation and other coils, and the electron guns.
Coordinate locations are discrete physical locations on the face of the cathode ray tube, or a physical area on the CRT screen.
Correction and driver circuitry are one or more of the following: digital to analog converters, interpolation engine, pulse width modulators and pulse density modulators, as well as summing amplifiers, oscillators, op-amps, inverters comparators or any other components necessary to produce and condition correction signals to synchronously apply to control circuitry to generate an aligned video image.
Correction control signals are correction factor signals that have been combined in a manner to be applied to either horizontal control circuitry, vertical control circuitry, coils, or electron gun circuitry.
Correction factor data includes the encoded digital bytes or any other form of data, such as term multipliers that adjust standardized transformation equations, that are representative of the amount of correction required to align a video signal at a particular physical location on a cathode ray tube to counteract distortion characteristics at that location. Correction factor data may include data from the gain matrix table, data relating to electron gun characteristics and/or data relating to geometry characteristics of the cathode ray tube.
Correction factor parameters include various geometry characteristics of the cathode ray tube including horizontal size, raster rotation, vertical size, horizontal center, vertical center, pin cushioning, vertical linearity, keystoning, convergence, etc., and various electron gun characteristics of the cathode ray tube including contrast, brightness, luminosity, focus, color balance, color temperature, electron gun cutoff, etc.
Correction factor signals are digital correction signals that have been integrated or filtered.
Correction signals are digital correction signals and correction factor signals.
A decoder is a device for generating an electronic signal in response to one or more data bytes that may include PWMs, PDMs, DACs, interpolation engines, on screen display chips, etc.
A deflection yoke is primarily two coils: a horizontal and a vertical coilxe2x80x94designating horizontal or vertical deflection of the electron beam through the CRT. Of these coils the major power consumer is the horizontal coil because it is driven at a high horizontal scan frequency, whereas the vertical coil is driven at the lower vertical scan frequency. Other coils may also deflect, rotate or reshape the electron beam or beams and be incorporated within the Deflection Yoke assembly.
A device is any apparatus which incorporates a cathode ray tube as part of its implementation.
Digital correction signals are signals that are generated by decoders, such as pulse width modulators, pulse density modulators, digital to analog converters, etc. in response to correction factor data.
A digital monitor board is a circuit board that includes one or more of the following: vertical control circuitry, horizontal control circuitry, electron gun control circuitry, correction and driver circuitry, a logic device, digital to analog converters, and a memory. A digital monitor board may comprise an actual chassis monitor board used with a particular monitor, an ideal chassis board, a chassis board that can be adjusted to match the characteristics or specifications of a particular monitor board, etc.
A digitized signal is any electrical signal that has a digital nature.
A direction can be up, down, left, right, clockwise anti-clockwise, brighter, dimmer, higher, lower, etc..
Discrete locations may be individual pixels on a cathode ray tube screen or may comprise a plurality of pixels on a cathode ray tube screen.
Distortion characteristics are the amount of any particular type of distortion as indicated by the distortion data measured at a number of different points on the cathode ray tube.
Distortion data is a measure of the amount of distortion that exists on a cathode ray tube with regard to the geometry characteristics of the tube, and/or electron gun characteristics of the tube. For example, distortion data can be measured as a result of misalignment of a video image or improper amplitude or gain of a video signal. Distortion data can be a quantitative measure of the deviation of correction factor parameters from a desired quantities value. Distortion data can be measured at coordinate locations on the cathode ray tube.
Driver signals are the electrical signals that are used to drive the deflection and control windings, and electron guns of the cathode ray tube.
Internal to raster is any part or section of a CRT raster as defined by its location on the raster.
Memory can be any desired storage medium including, but not limited to, EEPROMs, RAM, EPROMs, PROMs, ROMs, magnetic storage, magnetic floppies, bar codes, serial EEPROMs, flash memory, etc.
A non-volatile electronic storage device is an electrical memory device that is capable of storing data that does not require a constant supply of power.
A pattern generator is any type of video generator that is capable of generating a video signal that allows measurement of distortion data.
A processor is a logic device including, but not limited to state machines, micro-processors, etc.
A raster is all or part of the horizontal traces produced on the face of a CRT.
A rotation coil is a coil or winding used to rotate the raster in a CRT.
Rotational distortion characteristics are the amount of rotational distortion as indicated by the rotational distortion data measured at a number of different points on a cathode ray tube.
Rotational distortion data is a measure of the amount of rotational distortion that exists on a specific cathode ray tube with regard to the geometry characteristics of the tube. For example, rotational distortion data can be a result of misalignment of a video image on the face of the CRT. Rotational distortion data can be a quantitative measure of the deviation of rotational correction factor data from a desired quantitative value. Rotational distortion data can be measured at coordinate locations on the cathode ray tube.
Rotation driver signals are time dependent analog signals generated, by various methods, and applied synchronously to the CRT rotation coil to dynamically align traces of the CRT raster display.
A storage disk may include any type of storage device for storing data including magnetic storage devices such as floppy disks, optical storage devices, magnetic tape storage devices, magneto-optical storage devices, compact disks, etc.
A Variable Resistor is an apparatus capable of producing a changeable value of electrical resistance.
A video image is the displayed image that appears on the cathode ray tube screen that is produced in response to a video signal.
A video pattern is the video image of a pattern that appears on the cathode ray tube raster as a result of the video signal generated by the pattern generator.
A video signal is the electronic signal that is input to the electron guns of the cathode ray tube.
3. Description of Related Art
Almost all TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their images. In order to display an image on the entire screen, electronic circuits inside the TV use magnetic deflection coils (a horizontal deflection coil to move the beam from left to right and a vertical deflection coil to move the beam up and down) to move the electron beam in a xe2x80x9craster scanxe2x80x9d pattern across and down the screen. FIG. 1 illustrates a raster 100 on a conventional cathode ray tube where an electron beam paints one line across the screen from left to right 102a and then quickly moves back to the left side 104, and paints another horizontal line 102b, while continuously moving down slightly. When the beam reaches the right side of the bottom line 107, it is moved back to the upper left corner of the screen, as represented by line 106. When the beam is xe2x80x9cpaintingxe2x80x9d lines 102 it is on, and when it is xe2x80x9cretracingxe2x80x9d lines 104, it is off so that it does not leave a trail on the screen. The term horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from the bottom to the top of the raster 106.
However, in what is referred to as a sinusoidal or xe2x80x9czig-zagxe2x80x9d raster scan, there is no horizontal retrace (or flyback) since the image is painted both when the beam scans both left to right as well as right to left as shown in FIG. 2A. Ideally, for example, the electron beam paints a line 202 going from left to right and steps down an interval xe2x80x9cdxe2x80x9d and paints a next line 204 going from right to left. Unfortunately, conventional vertical amplifiers only produce a linearly changing magnetic flux in the vertical deflection coil, the resulting zig-zag raster is more like that shown in FIG. 2B, exhibiting a distorted raster when compared to the ideal raster scan of FIG. 2A.
Prior art has proposed a solution to make the scan lines parallel by using a wide bandwidth vertical amplifier driven with a stair stepped drive waveform to move the beam down the screen only during the time xe2x80x9cdxe2x80x9d (now the new horizontal blanking time) between the end of line 202 and the beginning of line 204. These approaches have proved, due in part to the high inductance of the vertical yoke, to be expensive, slow (requiring long horizontal blanking times), power hungry and unstable (producing vertical ringing at the beginning of the lines).
However a vastly simpler solution presents itself, when this downward tilting of each line is characterized as simple scan line rotational distortion.
According to the present invention, a method and apparatus for dynamically generating a rotational magnetic flux on a line by line basis in a cathode ray tube is described.
In an embodiment of the invention, a rotation coil is added to the deflection yoke assembly on a cathode ray tube. Accordingly, magnetic flux generated by this rotation or Line Twist Coil (LTC) is set going in the left to right direction but switches polarity when going from the right to left direction. It is this reverse twister flux that combines with the vertical deflection flux to step the scan line. Therefore, the LTC produces an additional flux field combining with the linear vertical flux field to produce the stepped line effect.
In one aspect of the invention, the LTC is a low inductance coil that can be driven by a line frequency signal in order to provide the desired dynamic compensation on a line by line basis.
In another aspect of the invention, the LTC can be incorporated directly into the deflection yoke, or otherwise closely coupled thereto.
In another embodiment of the invention, a method for dynamically compensating for the magnetic flux generated by the vertical deflection coil on a line by line basis in a zig-zag raster type cathode ray tube is described. A twister coil produces a counter flux field to the linear vertical flux field produced by a vertical deflection coil to produce a stepped line raster pattern.
In one aspect of the invention, a first varied magnetic flux is generated by a LTC when a raster scan is going in the left to right direction. When the raster scan goes form the right to the left direction, a second varied magnetic flux is generated by the LTC that combines with a vertical deflection flux to straighten the scan lines.